IMAP Capabilities

Capability names MUST either begin with "X" or be standard or
standards-track IMAP4rev1 extensions, revisions, or amendments
registered with IANA. A server MUST NOT offer unregistered or
non-standard capability names, unless such names are prefixed
with an "X".
Client implementations SHOULD NOT require any capability name other
than "IMAP4rev1", and MUST ignore any unknown capability names.